A display device having a plurality of pixels formed over a substrate has been known. As a typical example of such a display device, a liquid crystal display device (LCD), an electroluminescence display device (EL display device), and the like are represented.
An EL display device is a display device having a light-emitting element in each pixel where the light-emitting element has a structure in which a material exhibiting an electroluminescence (EL) phenomenon is sandwiched between a pair of electrodes. A light-emitting element using an organic compound as a material is called an organic EL element or an organic electroluminescence element and also called an organic light-emitting element diode (OLED) element due to its diode characteristics. A display device having a plurality of such light-emitting elements is called an organic EL display device. Since organic compounds are able to exhibit a variety of emission wavelengths due to their design diversity, colors with a variety of wavelengths can be obtained by appropriately selecting organic compounds. Furthermore, an organic EL display element has an excellent response property and is capable of giving an image with high contrast.
Generally, each pixel includes a light-emitting element and a driving transistor for supplying a current to drive the light-emitting element. After the driving transistor is formed over a substrate, the driving transistor is covered with an insulating film constructed with an organic resin material and the like over which the light-emitting element is formed. An electrical connection between the light-emitting element and the driving transistor is performed through an opening portion (contact hole) formed in the insulating film. As a typical example of such a structure, a display device with a structure disclosed in Japanese patent application publications No. 2001-148291 and 2012-23028 has been known.